1. Field of the Invention
The present invention relates to a cooling element for shaft furnaces provided with a refractory lining, particularly blast furnaces. The cooling element is made of copper or a low copper alloy and is provided with coolant ducts arranged in the interior of the element.
2. Description of the Related Art
Cooling systems for the steel jackets of shaft furnaces, particularly blast furnaces, are extensively described in "Stahl und Eisen", 106 (1986), No. 2, pages 205-210. In addition to cooling with so-called cooling boxes, in recent years cooling with cooling plates, so-called staves, of cast iron and copper has been used increasingly.
DE 39 25 280 discloses a cooling plate of grey cast iron in which the cooling ducts are formed by cooling tubes which are cast into the cast body. This cooling plate has the disadvantage that, for preventing carburization, a coating of the cooling tubes is required which impairs the thermal flux from the hot side of the cooling plate or stave through the stave body and the tube wall toward the cooling water. Accordingly, such staves frequently reached high temperatures in excess of 760.degree. C. at which decomposition of the pearlite occurs; cracks formed in the cast body and the cast material in front of the cooling tubes wears off even after a relatively short period of operation.
It has been attempted to achieve a longer durability of these staves of cast iron by casting a plurality of cooling tubes in the staves and to arrange these cooling tubes partially also in different planes parallel to the hot side. This made the staves of grey cast iron much more complicated and expensive, but the durability of the staves did not increase to the same extent.
A significant improvement were the so-called copper staves which are disclosed in DE 29 07 511 and are manufactured from rolled copper material, wherein the cooling ducts are produced by deep hole drilling parallel to the hot side. This makes possible an unimpeded thermal flux which is not impaired by any coating of the tubes. Copper staves of this type are significantly cooler on their hot sides than staves of grey cast iron, so that, contrary to staves of grey cast iron, a stable crust of burden material acting as insulation is formed on the hot side. This is the reason why copper staves, even though the thermal conductivity of this material is high, discharge less heat from a blast furnace than staves of grey cast iron.
Another advantage of the copper staves is the fact that they can be constructed thinner at about 150 mm than staves of grey cast iron at about 250 mm. Consequently, at a given size of the blast furnace, the useful volume is increased significantly when copper staves are used.
However, the decisive advantage of the copper staves as compared to staves of cast iron is the fact that they do not exhibit the formation of cracks because of the material properties and their surface wear is extremely low. In a long term experiment extending over more than ten years, a material loss of only 3 to 4 mm was observed. In the case of a rib height of 50 mm, this results in a computed service life of about 150 years which substantially exceeds the service life of the remaining blast furnace.
A disadvantage of the conventional copper staves is the fact that they are still constructed of relatively substantial solid material and, therefore, are heavy and expensive. The staves must be processed to a significant extent because of the necessary mechanical working on all sides, the cutting of grooves, the deep hole drilling and the welding of the pipe connections. The material removed by chip-removing processes constitutes a substantial portion of the total weight and can be sold only at a significantly lower price. Another disadvantage is the fact that when deep hole drilling is carried out in excess of 2 to 3 m depth, the duct diameters may not be less than a certain dimension because otherwise there is the danger that the drill runs off center. The cooling ducts produced in this manner are larger than necessary; the same is true for the quantity of cooling water because a minimum speed of about 1.5 m/sec is necessary for separating steam bubbles which may form at the tube wall as a result of the high thermal load. Consequently, the cooling water heating rates are uneconomically low.